1. Field of the invention
The present invention relates to a technique for improving the productivity of a semiconductor device, in particular, to a technique for preventing electrostatic discharge damage to a switching element such as a thin film transistor (TFT).
The term semiconductor device herein generally refers to a device that utilizes semiconductor characteristics for functioning the device. Accordingly, electro-optical devices (hereinafter referred to as display devices), semiconductor circuits, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
A technique for manufacturing a TFT comprising a semiconductor thin film (with a thickness of several hundreds to several thousands nm) formed on a substrate having an insulating surface has recently been developed. TFTs are applied to various semiconductor devices such as integrated circuits (ICs) and electro-optical devices, and quick development of TFTs as switching elements of the display devices or the like is demanded.
Recent semiconductor devices are finding their uses expanding as monitors, televisions, and the display devices for portable terminals. Accordingly, mass production of the semiconductor devices has become common.
FIG. 18 is a diagram showing an example of the entire circuit structure of a conventional semiconductor device. A large number of pixel cells are arranged in rows and columns, forming a pixel cell array in a pixel region 1701. Each of the pixel cells comprises a TFT, a transparent pixel electrode, a liquid crystal, and a capacitor storage.
A gate signal line side driving circuit 1711 comprises a shift register circuit 1707, a level shifter circuit 1708, a buffer circuit 1709, and a protective circuit 1710.
A source signal line side driving circuit 1712 comprises a shift register circuit 1702, a level shifter circuit 1703, a buffer circuit 1704, a sampling circuit 1705, and a pre-charge circuit 1706. Also, the pre-charge circuit 1706 may be positioned so as to face the shift register circuit 1702, the level shifter circuit 1703, the buffer circuit 1704, and the sampling circuit 1705 with the pixel region 1701 interposed therebetween.
In this semiconductor device, a crystalline semiconductor film is formed on an insulating surface, a gate insulating film is formed on the crystalline semiconductor film, and a gate electrode is formed on the gate insulating film in order to fabricate the thin film transistors. The thin film transistors are then covered with an interlayer insulating film, and contact holes are formed in the interlayer insulating film through dry etching and a metal wiring line is formed to electrically connect the thin film transistors with one another.
The above semiconductor device manufacturing process is characterized in that the pixel region 1701, the gate signal line side driving circuit 1711, and the source signal line side driving circuit 1712 are simultaneously formed through the same steps.
A description will be given with reference to FIGS. 19A to 19C on a method of manufacturing the conventional circuit, electrostatic generation, and damage to TFTs by static electricity. Crystalline semiconductor films 1803 and 1804 are formed first on an insulating surface. Next, an insulating film 1805 is formed so as to cover the crystalline semiconductor films 1803 and 1804. A gate signal line 1806 is then formed on the insulating film 1805. Through these three steps, a pixel TFT 1801 and a driving circuit TFT 1802 are formed simultaneously. (FIG. 19A)
An interlayer insulating film 1807 is formed so as to cover the pixel TFT 1801 and the driving circuit TFT 1802. In order to connect the pixel TFT 1801 to the driving circuit TFT 1802 electrically, contact holes 1808 and 1809 are formed by dry etching. It has been confirmed that static electricity is generated during the dry etching step, and that the generated static electricity moves from one contact hole to the gate signal line. An arrow shown in FIG. 19B indicates the movement of the static electricity generated in the contact hole of the pixel TFT toward the driving circuit TFT through the gate signal line. Reaching the driving circuit TFT, the static electricity damages the gate insulating film of the driving circuit TFT and then moves to the crystalline semiconductor film 1803. The driving circuit TFT 1802 is thus damaged by static electricity. (FIG. 19B) The conventional circuits are not capable of preventing damage to TFTs by static electricity that is generated and moves as described above.
FIG. 20 shows electrostatic discharge damage to a TFT near a pre-charge circuit in a conventional circuit. Static electricity generated in a pixel region moves along a source signal line to a contact hole 1903 on the upper end of the pixel region. The static electricity then moves to a contact hole 1904 in a drain portion of the pre-charge circuit. From the contact hole 1904 in the drain portion of the pre-charge circuit, the static electricity moves to a first signal line 1905 of the pre-charge circuit and then to a second signal line 1906 of the pre-charge circuit.
When the static electricity moves from the contact hole 1904 in the drain portion of the pre-charge circuit to the first signal line 1905 of the pre-charge circuit. an insulating film is damaged to break the pre-charge circuit. The conventional circuits are not capable of preventing damage to pre-charge circuits by static electricity that is generated and moves as described above.
Electrostatic discharge damage as described above may lead to display defects such as line defect and dot defect in panel display by a semiconductor device, thereby lowering its yield and reliability.
Upon manufacturing a semiconductor device, minute processing is required and dry etching that can provide excellent minute processing is indispensable especially upon forming a contact hole with a diameter of 3 μm in an interlayer insulating film.
A dry etching process includes the following steps (1) through (6) and the steps (2) through (6) are repeated.                (1) An etching gas (e.g., XeF2 or CF4) is introduced into a vacuum chamber and a high frequency voltage is applied between an upper electrode and a lower electrode to generate plasma.        (2) Reactive ions having positive electric charge from the generated plasma enter a surface of the interlayer insulating film at a right angle.        (3) The reactive ions adsorb to the surface of the interlayer insulating film.        (4) The reactive ions that has adsorbed to the surface of the interlayer insulating film react thereon to produce a reaction product.        (5) The reaction product leaves from the surface of the interlayer insulating film.        (6) The reaction product that has left from the surface of the interlayer insulating film is discharged.        
Electrostatic generation is a result of separating positive electric charge and negative electric charge from each other by a mechanical effect and therefore takes place between the surfaces of solids or between the surfaces of a solid and a liquid. Static electricity is generated also when a gas separates a solid surface and a liquid surface or when a solid or a liquid contains an ionized gas. Then static electricity could be generated in the above steps (3) and (5) and electrostatic generation cannot be avoided.
When a contact hole is opened in an interlayer insulating film through dry etching in particular, electrostatic discharge damage that takes place between the contact hole and a gate signal line is so great that silicon eliminates. The static electricity in this case is often observed to move from a contact hole to another contact hole.
A long gate signal line has an antenna effect that attracts static electricity, and electrostatic discharge damage takes place at both ends of the gate signal line.
Against electrostatic discharge damage, the conventional circuit is provided with a protective circuit 1710 that is placed in the gate signal line side driving circuit 1711. However, the protective circuit 1710 is the type that functions after a metal wiring line is formed and therefore has no effect of preventing damage to the TFT by static electricity generated when the contact holes are opened in the interlayer insulating film as a step included in the TFT manufacturing process. The static electricity thus moves along the gate signal line to the gate electrode of the TFT and damages the gate insulating film there before moving to the source signal line from a source or drain region of the TFT. In the manner described above, static electricity damages the TFTs in the gate signal line side driving circuit 1711 and in the pixel region 1701.
The static electricity generated in the pixel region during the dry etching for opening the contact holes in the interlayer insulating film further moves from the contact hole at the upper end of the pixel region 1701 to the contact hole in the drain portion of the pre-charge circuit 1706. Then the static electricity moves to the first signal line of the pre-charge circuit 1706 and to the second signal line of the pre-charge circuit. As the static electricity moves from the contact hole in the drain portion of the pre-charge circuit 1706 to the first signal line of the pre-charge circuit 1706, it damages the insulating film.
As described above, the conventional semiconductor device circuit is not capable of preventing damage to the TFTs in the gate signal line side driving circuit 1711, the pixel region 1701, and the source signal line driving circuit 1712 by the static electricity that is generated during dry etching for opening the contact holes in the interlayer insulating film. This leads to display defects such as line defect and dot defect in panel display of the semiconductor device, thereby lowering yield and reliability.